WhatsApp Backup to iCloud

Hi, I'm using an iPhone 6S. There is sufficient space on my iCloud, around 4.2 GB. My iCloud refuses to backup my WhatsApp. My WhatsApp is around 2.88 GB. Every time I begin to back up it starts and then stops between 2% and 7% and then gets stuck. I just bought a new iPhone and I wanted to backup my old phone before I started using my new one so that I could restore all my data. I've tried restarting and rebooting my phone. I've tried to connect to different networks. When i emailed the WhatsApp support regarding the same, they responding saying that there might be a problem with my iCloud account.